A conventional wind turbine generator system according to a prior art has been as follows. A windmill is connected to a synchronous generator and power is generated as the windmill is rotated by wind energy and so the synchronous generator is driven by the windmill. The alternating current power generated by the synchronous generator is converted into direct current power by a converter and further converted into the commercial-frequency alternating current power by an inverter, and then supplied to an electric power system. A wind turbine genarator system having the above construction has been disclosed, for example, in the Patent Document 1.
[Patent Document 1]
Japanese Patent Application Laid-Open Publication No. 2003-120504.